X-ray studies of dUMP HMase are being pursued to address what structural features determine whether addition of water (dUMP HMase) or hydride transfer (TS) occurs to the exocyclic methylene of 5-CH 2-dUMP. The high power and high flux of a synchrotron X-ray beam combined with rapid data collection capabilities make SSRL a valuable resource in surmounting the difficulties faced in the structural studies of dUMP HMase.